Device for thermoforming a plastic film

ABSTRACT

A thermoforming device heats a plastic film to a substantially uniform temperature, in order to impart a curved shape to said film. The device includes a chamber, the internal pressure of which is variable and which is closed by the film, a system for measuring a sag in the film arranged outside the chamber and opposite said film, and a system for heating the film. The heating system includes a hot air blowing unit which is suitable for producing a hot airflow flowing parallel to the film between two opposite edges of said film.

The invention relates to a device for thermoforming a plastic film.

It is known to preform a plastic film for application onto a curvedsurface of a substrate, in order to reduce the stresses produced in thisfilm when it is subsequently assembled with the substrate. In thismanner the film can be applied to the curved surface of the substratewithout excessive stretching or folds, tears, crushing, or scratchesbeing produced in the film by the tool used to apply it. To thispurpose, the plastic film is preformed using a thermoforming devicewhich may comprise:

-   -   a chamber with variable internal pressure, which has an opening        in a side of this chamber;    -   an attachment system for attaching the film around the opening,        which is adapted to hold a peripheral edge of the film firmly so        that the film closes off and seals the chamber;    -   a system for varying and controlling the pressure in the        chamber;    -   a measurement system for measuring a sag (“straight height”) of        the film which is held by the attachment system, this        measurement system being arranged outside the chamber and facing        the chamber opening; and    -   a system for heating the film, adapted to heat the film held by        the attachment system, and comprising a hot air blowing unit        arranged to produce a stream of hot air flowing externally to        the chamber and in contact with the film.

The film which is held by the attachment system is deformed, where itcrosses the opening of the chamber, in a deformation which varies withthe pressure that is created in the chamber. In this manner, the plasticfilm can be deformed while subjected to heat, with a deformationamplitude that is controlled while measuring the sag of the film in theopening of the chamber.

It is also known to place the substrate within the inside volume of thechamber, supporting it in a manner which is appropriate for controllingthe approach of the substrate to the plastic film while the film isstill being held in the opening of the chamber by the attachment system.Thus the thermoforming device also applies the film onto the substrate,which is particularly advantageous when compared to the number ofmanipulations of the film required to assemble the film and substratetogether.

To provide sufficiently precise measurements of the film sag, the sagmeasurement system must be positioned substantially in alignment withthe center of the chamber opening. For this reason, and when the insidevolume of the chamber is already occupied by the substrate, this sagmeasurement system is placed outside the chamber.

In existing thermoforming devices of the type considered, the system forheating the film is also located outside the chamber, next to the filmsag measurement system. This heating system commonly uses infraredradiation or blown hot air. The space that is then required, in the areaoutside the chamber in front of its opening, interferes with the heatingof the film. The temperature of the heated film then varies betweendifferent points of said film. The deformation of the film produced bythe thermoforming is then distributed in a manner which no longercorresponds to the desired shape. In particular, the distribution ofthis deformation is no longer rotationally invariant around an axisperpendicular to the side of the chamber which contains the opening, andwhich passes through the center of this opening. Certain of the defectsmentioned above then reappear when a film preformed in this manner isapplied onto the substrate, particularly when the substrate face issubstantially spherical in shape.

In particular, such thermoforming devices are used to preform a plasticfilm that is intended to be applied onto an optical lens. The qualityrequirements of the final optical component are then particularly high,especially when it is a component such as a spectacle lens or a lensblank. For this type of application, the end product is rejected whenstretching, folds, tears, crushing, or scratches in the film are visibleto the naked eye. Thus for optical and ophthalmic applications inparticular, the thermoforming devices currently in use areunsatisfactory.

One object of the invention therefore consists of providing athermoforming device which does not have the above disadvantages andwhich is compatible with the quality requirements of optical orophthalmic products.

To achieve this, the invention proposes an improvement to athermoforming device as described above, in which the blowing unit isadapted so that the stream of hot air flows parallel to the plastic filmfrom a first lateral side of the chamber opening to a second lateralside of this opening, said second side being opposite the first side.

Thus the hot air used to heat the plastic film can be brought to it froma lateral side of the chamber, which reduces the amount of space that isoccupied outside the chamber in front of the opening. The film sagmeasurement system can then be properly arranged in this space in directalignment with the film, to provide precise measurements of the filmsag.

In addition, given that the stream of hot air flows parallel to the filmbetween two lateral and opposite sides, the flow of this hot air can beparticularly stable and have a constant distribution in contact with theplastic film. The temperature of the heated film is then more uniform,so that the thermoforming gives the film a deformation which isdistributed in the desired manner. The subsequent application of thefilm onto the substrate no longer produces unacceptable defects in theend product, even when it is an optical or ophthalmic product such as acontact lens or spectacle lens.

Preferably, the thermoforming device is adapted so that the stream ofhot air forms a laminar flow, without turbulence, parallel to and incontact with the film, between the first and second lateral sides of thechamber opening. Such a tangential laminar flow of the stream of hot airon the plastic film avoids the deformed plastic film being pushed by thestream of hot air towards the second lateral side. Thus the deformationof the film is distributed more isotropically around the central axisperpendicular to the opening.

The following improvements of the invention further improve thedistribution of the stream of hot air on the film, and therefore thedistribution of the deformation applied to the film by thethermoforming.

In a first improvement, the device may comprise a ring which extendsaround the chamber opening, outside said chamber, with a face that isslanted relative to a midplane of the opening, and a lower internal edgeof this slanted face which is arranged to come in contact with or inproximity to the film held by the attachment system, along at least aportion of the peripheral edge of the film. The blowing unit is thenarranged to produce the stream of hot air from an upper external edge ofthe slanted face, on the first lateral side of the chamber opening. Thusthe stream of hot air is guided to or close to the plastic film by theslanted face of the ring, avoiding the presence of an area of stationaryair in a recessed angle between the ring and the plastic film.

In a second improvement of the invention, the hot air blowing unit maycomprise a nozzle which has a hot air outlet directed towards thechamber opening, on the first lateral side of the opening, and thenozzle may comprise a deflector with an oblique deflecting face, whichis arranged to redirect the stream of hot air towards the film held bythe attachment system. Thus the stream of hot air is also guided priorto its arrival onto the plastic film, in order to better establish astable and laminar flow of said air.

When these two improvements of the invention are implemented together,the angle of the slanted face of the ring and the angle of thedeflecting face may have a difference that is less than 20°, andpreferably less than 10°, these two angles being measured relative to anaxis which is perpendicular to the side of the chamber, and in amidplane of the opening which connects the two lateral sides. Thus thering and the deflector together form a continuous and effective guidefor the stream of hot air until it reaches the plastic film.

In a third improvement of the invention, the nozzle may comprise innerwalls arranged longitudinally to partition the nozzle into separatechannels which guide the hot air towards the nozzle outlet.

Other features and advantages of the invention will be apparent from thefollowing description of a non-limiting example, with reference to theattached drawings in which:

FIG. 1 is a general schematic view of a thermoforming device of theinvention;

FIGS. 2 a and 2 b are more detailed perspective and side views of aportion of the thermoforming device of FIG. 1;

FIGS. 3 a and 3 b are respectively perspective and side views of anozzle used in the thermoforming device of FIGS. 1, 2 a and 2 b;

FIG. 3 c is a perspective view of a deflector of the nozzle of FIGS. 3 aand 3 b; and

FIGS. 4 a and 4 b are cross-sectional and perspective views of a ringused in the thermoforming device of FIGS. 1, 2 a and 2 b.

For clarity, the dimensions of the elements represented in these figuresare not necessarily in proportion to the actual dimensions, or to therelative proportions between the actual dimensions. In addition, thesame references are used in the different figures to denote identicalelements.

The invention is now described in detail in the context of an ophthalmicapplication, in which a plastic film is applied onto a blank for aspectacle lens. The lens blank may be of any type, of mineral, organic,or even a hybrid material. It has a diameter which is usually about 60mm (millimeters), and a definitive face onto which the plastic film isto be applied. This face may be convex or concave. For the applicationof the film, the shape of the face of the lens blank can be compared toa portion of a sphere, even if it is a complex surface without an axisof symmetry, for example for a progressive lens. The plastic film mayalso be of any type, consisting of one or more materials which can beformed using heat. It may have any function, or multiple functions suchas anti-reflective, anti-soiling, anti-scratch, anti-shock,anti-fogging, photochromatic, etc. Possibly the plastic film may have acomplex, multilayer or cellular structure. In all cases, “film” isunderstood to mean the element which is initially flat and is intendedto be preformed and then applied onto the lens blank, regardless of itsinternal structure.

A plastic film thermoforming device of FIGS. 1, 2 a, and 2 b comprises:

-   -   a chamber 100 with variable internal pressure, with an opening O        which may be located in an upper side of the chamber;    -   a system for attaching the plastic film 200 across the opening        O, in a manner that closes off the opening O and forms a seal;    -   a system for varying and controlling the pressure inside the        chamber 100, which may consist of a pump 101, denoted P, and a        pressure sensor which indicates the pressure inside the chamber        100, not represented,    -   a system 300 for measuring a deformation of the plastic film 200        in the opening O; and    -   a system for heating the film 200, consisting of a unit for        producing and blowing hot air.

It is assumed in the following description that the chamber 100 isrotated so that the opening O is substantially horizontal and appears onthe top of the chamber 100.

When the thermoforming device is also able to apply the film 200 ontothe lens blank after this film has been preformed by thermoforming, thechamber 100 may additionally contain a holder 102 onto which the lensblank is placed. This latter is denoted by the reference 400 in FIG. 1.Advantageously, the lens blank 400 may be placed on the holder 102 sothat the face S₄₀₀ of the blank onto which the film 200 is to be appliedis facing the film 200. As an illustration, the face S₄₀₀ may be theconvex face of the spectacle lens. Possibly, the holder 102 may beadapted to move the lens blank 400 towards the film 200, in particularto raise the blank until it comes into contact with the film 200. Tothis purpose, the holder 102 may have a piston 103 which can be raisedin a controlled manner inside a cylinder 104, for example usinghydraulic means 105.

The system for attaching the plastic film 200 in the opening O may becombined with one of the improvements of the invention, as will bedescribed below.

As the film 200 is plastic, when it has been previously heated, itdeforms across the opening O, towards the outside or towards the insideof the chamber 100 depending on whether the chamber has been brought bymeans of the pump 101 to a negative or positive pressure relative to theoutside. The amplitude of this deformation can be characterized by thesag f of the film 200 at a central point of the opening O. This sag f isdefined relative to the initial position of the film 200 before varyingthe pressure in the chamber 100. For most applications, the initialshape of the film 200 is flat.

The measurement system 300 which measures the deformation of the plasticfilm 200 is arranged above the center of the opening O, outside thechamber 100. In this position, it allows precise measurements of the sagf. The system 300 can comprise:

-   -   a detection head 301, which is adapted for receiving a signal R        from a central portion of the film 200 held by the attachment        system;    -   a detection head 301 displacement system 303, which is adapted        for moving the head 301 closer or further away relative to the        central portion of the film 200;    -   a control system 304 for the displacement system 303, which is        adapted for activating the system 303 in a manner that maintains        a constant amplitude of the signal R received by the detection        head 301; and    -   a system 305 adapted for reading a position of the detection        head 301.

Such a measurement system 303 can measure the sag f of the film 200 atany moment, particularly in a continuous manner, while the film isheated. It also allows measuring the sag f using a signal of a constantamplitude, when this signal is received by the detection head 301. Themeasurement of the sag f then corresponds to the displacement producedby the system 303. It is read on the system 305. Particularly precisemeasurements of the sag f can thus be obtained. The signal R received bythe detection head 301 can be produced by a separate emitting head 302.In this case, the two heads, the emitting head 302 and detection head301, are moved simultaneously by the system 303. Alternatively,depending on to the measurement technology that is used, the detectionhead 301 may also have the emission function. Preferably, the system 300makes use of the reflection of light or ultrasound. The signal R is thena light beam or an ultrasound beam that is reflected on the film 200.

The unit for producing and blowing hot air may comprise, depending onthe direction of the air flow:

-   -   one or more apertures 20 for the intake of a stream of air F₀;    -   a system for accelerating the stream F₀, which may comprise a        propeller 30 driven by a motor 31, and which produces an        accelerated stream of air F₁;    -   a system for heating the air in the stream F₁, which may consist        of a heating resistor 40;    -   a duct segment 50, which brings the stream of hot air F₁ to a        nozzle 60; and    -   the nozzle 60, which directs a stream of hot air F₂ issuing from        the stream F₁, through an outlet 61 of the nozzle and in the        direction of the plastic film 200.

The outlet 61 of the nozzle 60 is located on a lateral side C₁ of theopening O, outside the chamber 100, and is directed towards a side C₂ ofthe opening O, which is diametrically opposite the side C₁. In thismanner, the stream of hot air F₂ passes above and in contact with allthe film 200 which is exposed in the opening O.

The nozzle 60 has the additional function of giving a constant andlaminar flow to the stream of hot air F₂, with a uniform flowdistribution through the outlet 61. To this end, the nozzle 60 mayadvantageously have internal walls, which are arranged longitudinally inthe stream of hot air F₂, and which partition the nozzle 60 intoseparate channels. Such internal partitions are denoted 63 in FIG. 3 a.

Advantageously, to further improve the uniformity of the transversedistribution of the stream of hot air in the outlet 61, an additionaldistribution element 59 may be placed in the blowing unit, upstream fromthe nozzle 60. Such an element is adapted to produce a loss of airpressure in order to modify the distribution of the flow in the nozzle60. The element 59 for distributing the stream of hot air may comprisein particular a grid, a perforated plate, or a compacted permeableblock.

As shown in FIGS. 4 a and 4 b, the thermoforming device mayadvantageously comprise a ring 1 which surrounds the opening O of theenclosure 100, above the plastic film 200. Such a ring 1 may have anupper face S₁ which is slanted, relative to a midplane of the opening O,towards the center of this opening and towards the inside of the chamber100. For example, the face S₁ may be tapered and continuous around theopening O of the chamber 100. In this case, it has a rotational symmetryaround an axis A-A perpendicular to the side of the chamber 100 whichhas the opening O, said axis passing through a center of this opening.This slanted face S₁ allows reducing, or even eliminating, a heightdifference between a lower edge of the outlet 61 of the nozzle 60 andthe face of the plastic film 200 which is facing the outside of thechamber 100. Such a height difference would form a step likely toproduce turbulence in the stream of hot air F₂ at the places where thisstreams comes in contact with the film 200 and where it leaves it,meaning at sides C₁ and C₂. Put another way, the slanted face S₁ guidesthe stream of hot air F₂ in the lower portion of the stream. To thispurpose, the slanted face S₁ of the ring 1 has an lower internal edge B₁which is intended to come in contact with or near the film 200, and anupper external edge B₂ which is intended to be near the outlet 61 of thenozzle 60. Preferably, the slanted face S₁ forms an angle α which isbetween 25° (degrees) and 90°, or better yet between 65° and 80°,relative to the axis A-A in the midplane of the opening O which connectsthe sides C₁ and C₂.

In a particularly advantageous embodiment of the invention, the ring 1may additionally act as a clamp holding the film 200 onto the chamber100. In this case, the ring 1 is part of the attachment system whichattaches the film to the chamber, around the opening O. It then has thetwo functions of guiding the stream of hot air F₂ and attaching the film200 to close off the chamber 100 and seal it. Alternatively, the ring 1may only cover at least a part of the system for attaching the film 200to the chamber 100 around the opening O.

With reference to FIGS. 3 a to 3 c, the nozzle 60 has a deflector 62 inits outlet 61. When the nozzle 60 is assembled with the chamber 100equipped with the ring 1 (FIGS. 1, 2 a and 2 b), the deflector 62 guidesthe stream of hot air F₂ in the upper portion of the stream, as thestream F₂ approaches the film 200. Thus the face S₁ of the ring 1 andthe deflector 62 are complementary in guiding the stream F₂. Thedeflector 62 has a deflecting face S₆₂ which is in contact with thestream of hot air F₂, and which is slanted to redirect the stream F₂towards the film 200. Preferably, this deflecting face S₆₂ is slanted atan angle β which is between 25° and 90°, or better yet between 60° and75°, relative to the axis A-A, in the midplane of the opening whichconnects the sides C₁ and C₂.

Advantageously, the angle α of the slanted face S₁ of the ring 1,measured relative to the axis A-A, and the angle β of the deflectingface S₆₂ of the deflector 62, also measured relative to the axis A-A,have a difference which is less than 20°, and is preferably less than10°. Thus the ring 1 and the deflector 62 together form a complete guidefor the stream of hot air F₂, at the top and at the bottom, so that thestream F₂ has a laminar and stable flow.

Also, the deflecting face S₆₂ may advantageously have an end edge B₆₂which extends substantially parallel to a portion of the peripheral edgeof the opening O of the chamber 100.

When the film 200 is to be applied to the lens blank 400, the opening Omay be circular with a diameter D of about 77 mm (FIGS. 4 a and 4 b).The outlet 61 of the nozzle 60 is near the peripheral edge of theopening O, and may follow this edge for an angular section θ of about150° (FIGS. 3 a and 3 c). The slanted face S₁ of the ring 1 may be acone of axis A-A having a half-angle at the apex α that is substantiallyequal to 72.5°.

The nozzle 60 may have a hot air inlet which is parallel to the plane ofthe opening O, and is circular with a radius of 31 mm for example. Thehot air distribution element 59 may then be placed in this inlet of thenozzle 60. The length L of the nozzle 60 may be about 220 mm, in acentral sectional plane of the nozzle (FIG. 3 b). In addition, thenozzle 60 may be internally divided by three internal walls 63, formingfour guide channels for the hot air which have substantially identicaltransverse cross-sections. The height H of the walls 63, measuredparallel to the axis A-A and corresponding to the depth of the channels,may be equal to 25.5 mm, for example.

Lastly, the deflecting face S₆₂ may be a portion of a cylinder of radius77.25 mm and having an axis of rotation sloped by the angle β relativeto the axis A-A. For example, the angle β can be equal to 67.5° . Theoutlet 61 may have a height h of about 5.6 mm, and the deflecting faceS₆₂ may have a length I of about 13.5 mm, measured in projectionparallel to the plane of the opening O.

The hot air may travel in the nozzle 60 at a temperature which issubstantially equal to 700° C. and at a flow rate which is between 400and 700 l/min (liters per min). Under these conditions, the meantemperature of the heated film 200 may be about 140°, with thedeviations from this mean value, at different locations in the openingO, being less than 10° C. or even less than 5° C.

Once the plastic film 200 is heated in this way, the pressure in thechamber 100 can be varied to preform the film, with real-time monitoringand controlling of the sag increase.

It is understood that the invention can be implemented by modifying someaspects of the embodiment described above, while maintaining at leastsome of the listed advantages. In particular, the numerical values whichhave been given, particularly the dimensions, are illustrative and canbe modified according to the size of the plastic film to bethermoformed.

The invention claimed is:
 1. A device for thermoforming a plastic film,comprising: a chamber configured to have variable internal pressure, andhaving an opening in a side of said chamber; an attachment systemconfigured to attach the film around said opening, and hold a peripheraledge of the film firmly so that said film closes off and seals thechamber; a pressure control system configured to vary and control thepressure in the chamber, and cause the film held by the attachmentsystem to deform in the chamber opening into a deformation which variesas a function of said pressure; a measurement system configured tomeasure a sag of the film held by the attachment system, saidmeasurement system being arranged outside the chamber and facing theopening of said chamber; and a heating system configured to heat saidfilm held by the attachment system, and comprising a hot air blowingunit arranged to produce a stream of hot air flowing externally to thechamber and in contact with the film, wherein the blowing unit isconfigured to cause the stream of hot air to flow parallel to the filmfrom a first lateral side of the opening of the chamber to a secondlateral side of said opening, said second side being opposite said firstside, wherein the measurement system compromises: a detection headconfigured to receive a signal from a central portion of the film heldby the attachment system; a detection head displacement systemconfigured to move said detection head closer or furter away relative tothe central portion of the film; a control system configured to activatesaid displacement system in a manner that maintains a constant amplitudeof the signal received by said detection head; and a reading systemconfigured to read a position of the detection head.
 2. The deviceaccording to claim 1, wherein the blowing unit is configured to causethe stream of hot air to form a laminar flow parallel to and in contactwith the film, between the first and the second lateral sides of theopening of the chamber.
 3. The device according to claim 1, comprising aring extending around the opening of the chamber, outside said chamber,with a slanted face that is slanted relative to a midplane of theopening, and with a lower internal edge of said slanted face beingarranged to come in contact with or in proximity to the film held by theattachment system, along at least a portion of the peripheral edge ofsaid film, and with the blowing unit being arranged to produce thestream of hot air from an upper external edge of the slanted face, onthe first lateral side of the chamber opening.
 4. The device accordingto claim 3, wherein the slanted face of the ring is tapered andcontinuous around the opening of the enclosure.
 5. The device accordingto claim 3, wherein the slanted face of the ring forms an angle which isbetween 25° and 90° relative to an axis perpendicular to the side of thechamber, and in the midplane of the opening connecting the first andsecond lateral sides.
 6. The device according to claim 3, wherein thering is configured as a clamp holding the film onto the chamber, and isa part of the attachment system for said film or at least partiallycovers the attachment system for the film.
 7. The device according toclaim 1, wherein the hot air blowing unit comprises a nozzle having ahot air outlet directed towards the opening of the chamber, on the firstlateral side of said opening, said nozzle comprising a deflector with anoblique deflecting face, arranged to redirect the stream of hot airtowards the film held in place by the attachment system.
 8. The deviceaccording to claim 7, wherein the deflecting face is slanted at an anglewhich is between 25° and 90° relative to an axis perpendicular to theside of the chamber, and in the midplane of the opening connecting thefirst and second lateral sides.
 9. The device according to claim 7,wherein the deflecting face has an end edge which extends parallel to aportion of a peripheral edge of the opening of the chamber.
 10. Thedevice according to claim 8, comprising a ring extending around theopening of the chamber, outside said chamber, with a slanted face thatis slanted relative to a midplane of the opening, and with a lowerinternal edge of said slanted face being arranged to come in contactwith or in proximity to the film held by the attachment system, along atleast a portion of the peripheral edge of said film, and with theblowing unit being arranged to produce the stream of hot air from anupper external edge of the slanted face, on the first lateral side ofthe chamber opening, wherein: the slanted face of the ring forms anangle which is between 25° and 90° relative to the axis perpendicular tothe side of the chamber, and in the midplane of the opening connectingthe first and second lateral sides, and the angle of the slanted face ofthe ring, relative to the axis perpendicular to the side of the chamberdiffers by less than 20° from the angle of the deflecting face of thedeflector relative to said axis.
 11. The device according to claim 7,wherein the nozzle comprises internal walls arranged longitudinally topartition said nozzle into separate channels which guide the hot airtowards the outlet.
 12. The device according to claim 11, wherein theblowing unit additionally comprises a distribution element arrangedupstream from the nozzle and configured to distribute the stream of hotair and modify the distribution of said stream in said nozzle byproducing a loss of air pressure.
 13. The device according to claim 12,wherein the element for distributing the stream of hot air comprises agrid, a perforated plate, or a compacted permeable block.
 14. The deviceaccording to claim 1, wherein the measurement system is configured touse reflection of light or is configured to use ultrasound.